Not Applicable.
1. Field of the Invention
The present invention relates to coating compositions, and more particularly but not by way of limitation, to coating compositions which, when cured, provide substantially transparent coatings having enhanced abrasion resistance. In one aspect, the present invention relates to a coating composition having improved stability wherein the coating compositions are derived from aqueous-organic solvent mixtures containing effective amounts of epoxy functional silanes, tetrafunctional silanes and multifunctional compounds such as multifunctional carboxylic acids, multifunctional anhydrides, and mixtures thereof.
2. Description of Prior Art
The prior art is replete with compositions which, when applied to substrates and cured, provide transparent, abrasion resistant coatings for the substrates. Such coatings are especially useful for polymeric substrates where it is highly desirable to provide substrates with abrasion resistant surfaces, with the ultimate goal to provide abrasion resistant surfaces which are comparable to glass. While the compositions of the prior art have provided transparent coating compositions having improved abrasion resistant properties, such prior art compositions are generally lacking when compared to glass. Thus, a need has long existed for improved compositions having improved stability and which, when applied to a substrate, such as a polymeric substrate, and cured provide transparent, highly abrasion resistant coatings. It is to such compositions and processes by which such compositions are manufactured and applied to substrates that the present invention is directed.
The present invention provides compositions having improved stability which, when applied to a variety of substrates and cured, form transparent coatings having superior abrasion resistant properties. Broadly, the coating compositions of the present invention comprise an aqueous-organic solvent mixture containing from about 10 to about 99.9 weight percent, based on the total solids of the composition, of a mixture of hydrolysis products and partial condensates of an epoxy functional silane and a tetrafunctional silane and from about 0.1 to about 30 weight percent, based on the total solids of the composition, of a multifunctional compound selected from the group consisting of multifunctional carboxylic acids, multifunctional anhydrides and combinations thereof. The epoxy functional silane and the tetrafunctional silane are present in the aqueous-organic solvent mixture in a molar ratio of from about 0.1:1 to about 5:1. The coating compositions of the present invention may further include from about 0.1 to about 50 weight percent of a mixture of hydrolysis products and partial condensates of one or more silane additives, based on the total solids of the composition, and/or an amount of colloidal silica or a metal oxide or combinations thereof equivalent to from about 0.1 to about 50 weight percent solids, based on the total solids of the composition.
It is an object of the present invention to provide coating compositions having improved stability which form transparent coatings upon curing. It is a further object of the present invention to provide stable coating compositions which form transparent coatings upon curing having improved abrasion resistance.
Other objects, advantages and features of the present invention will become apparent upon reading the following detailed description in conjunction with the appended claims.
The present invention relates to coating compositions having improved stability which, when applied to a variety of substrates and cured, form substantially transparent abrasion resistant coatings having a Bayer number of at least 5 when tested in accordance with the variation of the Oscillating Sand Test (ASTM F735-81) hereinafter described.
For testing abrasion resistance of coated substrates, any of a number of quantitative test methods may be employed, including the Taber Test (ASTM D-4060), the Tumble Test and the Oscillating Sand Test (ASTM F735-81). In addition, there are a number of qualitative test methods that may be used for measuring abrasion resistance, including the Steel Wool Test and the Eraser Test. In the Steel Wool Tests and the Eraser Test, sample coated substrates are scratched under reproducible conditions (constant load, frequency, etc.). The scratched test samples are then compared and rated against standard samples. A semi-quantitative application of these test methods involves the use of an instrument, such as a Spectrophotometer or a Colorimeter, for measuring the scratches on the coated substrate as a haze gain.
The measured abrasion resistance of a cured coating on a substrate, whether measured by the Bayer Test, Taber Test, Steel Wool Test, Eraser Test, Tumble Test, etc. is a function, in part, of the cure temperature and cure time. In general, higher temperatures and longer cure times result in higher measured abrasion resistance. Normally, the cure temperature and cure time are selected for compatibility with the substrate; although, sometimes less than optimum cure temperatures and cure times are used due to process and/or equipment limitations. It will be recognized by those skilled in the art that other variables, such as coating thickness and the nature of the substrate, will also have an effect on the measured abrasion resistance. In general, for each type of substrate and for each coating composition there will be an optimum coating thickness. The optimum cure temperature, cure time, coating thickness, and the like, can be readily determined empirically by those skilled in the art.
Within the Ophthalmic Industry, the Oscillating Sand Test is presently the most widely used and accepted method for measuring abrasion resistance. Since the original ASTM application of the Oscillating Sand Test was for testing flat polymeric sheets, the test method has necessarily been modified for use with ophthalmic lenses. There is currently no ASTM accepted standard (or other industry standard) for this test as applied to ophthalmic lenses; therefore, there are a number of basic variations of the Oscillating Sand Test in practice.
In one particular variation of the Oscillating Sand Test, a sand cradle is modified to accept coated sample lenses and uncoated reference lenses. Typically, poly(diethylene glycol-bis-allyl carbonate) lenses, hereinafter referred to as ADC lenses, are used as the reference lenses. The lenses are positioned in the cradle to allow a bed of abrasive material, either sand or a synthetically prepared metal oxide, to flow back and forth across the lenses, as the cradle oscillates back and forth at a fixed stroke, frequency and duration.
In the test method employed to determine the abrasion resistance of the coating compositions of the present invention, a commercially available sand sold by CGM, Inc., 1463 Ford Road, Bensalem, Pa., was used as the abrasive material. In this test, 877 grams of sifted sand (600 ml by volume) was loaded into a 9 {fraction (5/16)}xe2x80x3xc3x976 xc2xexe2x80x3 cradle fitted with four lenses. The sand was sifted through a #5 Mesh screen (A.S.T.M.E.-11 specification) and collected on a #6 Mesh screen. Each set of four lenses typically two ADC lenses and two coated lenses, was subjected to a 4 inch stroke (the direction of the stroke coinciding with the 9 {fraction (5/16)}xe2x80x3 length of the cradle) at a frequency of 300 strokes per minute for a total of 3 minutes. The lens cradle was then repositioned by turning 180 degrees and then subjected to another 3 minutes of testing. Repositioning of the cradle was used to reduce the impact of any inconsistencies in the oscillating mechanism. The ADC reference lenses used were Silor 70 mm plano FSV lenses, purchased through Essilor of America, Inc. of St. Petersburg, Fla.
The haze generated on the lenses was then measured on a Gardner XL-835 Colorimeter. The haze gain for each lens was determined as the difference between the initial haze on the lenses and the haze after testing. The ratio of the haze gain on the ADC reference lenses to the haze gain on the coated sample lenses was then reported as the resultant abrasion resistance of the coating material. A ratio of greater than 1 indicates a coating which provides greater abrasion resistance than the uncoated ADC reference lenses. This ratio is commonly referred to as the Bayer ratio, number or value; the higher the Bayer number, the higher the abrasion resistance of the coating. Coatings produced by curing the coating compositions of the present invention, when tested using the Oscillating Sand Test method as described above, coated on either polycarbonate or on ADC lenses, have been shown to provide Bayer numbers which exceed 5. For testing coated samples, samples coated on ADC lenses were cured at a temperature of 120xc2x0 C. for a period of 3 hours. Samples coated on polycarbonate lenses were cured at a temperature of 129xc2x0 C. for a period of 4 hours.
One who is skilled in the art will recognize that: (a) The descriptions herein of coating systems which contain epoxy functional silanes, tetrafunctional silanes, silane additives which do not contain an epoxy functional group, and the multifunctional component refer to the incipient silanes and multifunctional components from which the coating system is formed, (b) when the epoxy functional silanes, tetrafunctional silanes, and silane additives which do not contain an epoxy functional group, are combined with the aqueous-solvent mixture, partial or fully hydrolyzed species will result, (c) the resultant fully or partially hydrolyzed species will combine to form mixtures of multifunctional oligomeric siloxane species, (d) these oligomers may or may not contain both pendant hydroxy and pendant alkoxy moieties and will be comprised of a silicon-oxygen matrix which contains both silicon-oxygen siloxane linkages and silicon-oxygen multifunctional component linkages, (e) these are dynamic oligomeric suspensions that undergo structural changes which are dependent upon a multitude of factors including; temperature, pH, water content, catalyst concentration, and the like.
The coating compositions of the present invention comprise an aqueous-organic solvent mixture containing from about 10 to about 99.9 weight percent, based on the total solids of the composition, of a mixture of hydrolysis products and partial condensates of an epoxy functional silane and a tetrafunctional silane and from about 0.1 to about 30 weight percent, based on the total solids of the composition, of a multifunctional compound selected from the group consisting of multifunctional carboxylic acids, multifunctional anhydrides, and combinations thereof. It will be recognized by those skilled in the art that the amount of epoxy functional silane and the amount of tetrafunctional silane employed to provide the mixture of hydrolysis products and partial condensates of the epoxy functional silane and the tetrafunctional silane can vary widely and will generally be dependent upon the properties desired in the coating composition, the coating formed by curing the coating composition, as well as the end use of the substrate to which the coating composition is applied. Generally, however, desirable results can be obtained where the epoxy functional silane and the tetrafunctional silane are present in the aqueous-solvent mixture in a molar ratio of from about 0.1:1 to about 5:1. More desirably, the epoxy functional silane and the tetrafunctional silane are present in the aqueous-solvent mixture in a molar ratio of from about 0.1:1 to about 3:1.
While the presence of water in the aqueous-organic solvent mixture is necessary to form hydrolysis products of the silane components of the mixture, the actual amount can vary widely. Essentially enough water is needed to provide a substantially homogeneous coating mixture of hydrolysis products and partial condensates of the epoxy functional silane and the tetrafunctional silane which, when applied and cured on an article, provides a substantially transparent coating with a Bayer number of at least 5 when using the method hereinbefore described. It will be recognized by those skilled in the art that this amount of water can be determined empirically.
The solvent constituent of the aqueous-organic solvent mixture of the coating compositions of the present invention can be any solvent or combination of solvents which is compatible with the epoxy functional silane, the tetrafunctional silane and the multifunctional component. For example, the solvent constituent of the aqueous-organic solvent mixture may be an alcohol, an ether, a glycol or a glycol ether, a ketone, an ester, a glycolether acetate and mixtures thereof. Suitable alcohols can be represented by the formula ROH where R is an alkyl group containing from 1 to about 10 carbon atoms. Some examples of alcohols useful in the application of this invention are methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, tertiary butanol, cyclohexanol, pentanol, octanol, decanol, and mixtures thereof.
Suitable glycols, ethers, glycol ethers can be represented by the formula R1xe2x80x94(OR2)xxe2x80x94OR1 where x is 0, 1, 2, 3 or 4, R1 is hydrogen or an alkyl group containing from 1 to about 10 carbon atoms and R2 is an alkylene group containing from 1 to about 10 carbon atoms and combinations thereof.
Examples of glycols, ethers and glycol ethers having the above-defined formula and which may be used as the solvent constituent of the aqueous-organic solvent mixture of the coating compositions of the present invention are di-n-butylether, ethylene glycol dimethyl ether, propylene glycol dimethyl ether, propylene glycol methyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, dipropylene glycol dimethyl ether, tripropylene glycol dimethyl ether, ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol dibutyl ether, ethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol dimethyl ether, ethylene glycol ethyl ether, ethylene glycol diethyl ether, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, butylene glycol, dibutylene glycol, tributylene glycol and mixtures thereof. In addition to the above, cyclic ethers such as tetrahydrofuran and dioxane are suitable ethers for the aqueous-organic solvent mixture.
Examples of ketones suitable for the aqueous-organic solvent mixture are acetone, diacetone alcohol, methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone and mixtures thereof.
Examples of esters suitable for the aqueous-organic solvent mixture are ethyl acetate, n-propyl acetate, n-butyl acetate and combinations thereof.
Examples of glycolether acetates suitable for the aqueous-organic solvent mixture are propylene glycol methyl ether acetate, dipropylene glycol methyl ether acetate, ethyl 3-ethoxypropionate, ethylene glycol ethyl ether acetate and combinations thereof.
The epoxy functional silane useful in the formulation of the coating compositions of the present invention can be any epoxy functional silane which is compatible with the tetrafunctional silane and the multifunctional component of the coating composition and which provides a coating composition which, upon curing, produces a substantially transparent, abrasion resistant coating having a Bayer number of at least about 5 when employing the test method hereinbefore described. Generally, such epoxy functional silanes are represented by the formula R3xSi(OR4)4xe2x88x92x where x is an integer of 1, 2 or 3, R3 is H, an alkyl group, a functionalized alkyl group, an alkylene group, an aryl group, an alkyl ether, and combinations thereof containing from 1 to about 10 carbon atoms and having at least 1 epoxy functional group, and R4 is H, an alkyl group containing from 1 to about 5 carbon atoms, an acetyl group, a xe2x80x94Si(OR5)3xe2x88x92yR6y group where y is an integer of 0, 1, 2, or 3, and combinations thereof where R5 is H, an alkyl group containing from 1 to about 5 carbon atoms, an acetyl group, or another xe2x80x94Si(OR5)3xe2x88x92yR6y group and combinations thereof, and R6 is H, an alkyl group, a functionalized alkyl group, an alkylene group, an aryl group, an alkyl ether, and combinations thereof containing from 1 to about 10 carbon atoms which may also contain an epoxy functional group.
Examples of such epoxy functional silanes are glycidoxymethyltrimethoxysilane, 3-glycidoxypropyltrihydroxysilane, 3-glycidoxypropyldimethylhydroxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropyldimethoxymethylsilane, 3-glycidoxypropyldimethylmethoxysilane, 3-glycidoxypropyltributoxysilane, 1,3-bis(glycidoxypropyl)tetramethyldisiloxane, 1,3-bis(glycidoxypropyl)tetramethoxydisiloxane, 1,3-bis(glycidoxypropyl)-1,3-dimethyl-1,3-dimethoxydisiloxane, 2,3-epoxypropyltrimethoxysilane, 3,4-epoxybutyltrimethoxysilane, 6,7-epoxyheptyltrimethoxysilane, 9,10-epoxydecyltrimethoxysilane, 1,3-bis(2,3-epoxypropyl)tetramethoxydisiloxane, 1,3-bis(6,7-epoxyheptyl)tetramethoxydisiloxane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, and the like.
The tetrafunctional silanes useful in the formulation of the coating compositions of the present invention are represented by the formula Si(OR7)4 where R7 is H, an alkyl group containing from 1 to about 5 carbon atoms and ethers thereof, an (OR7) carboxylate a xe2x80x94Si(OR8)3 group where R8 is a H, an alkyl group containing from 1 to about 5 carbon atoms and ethers thereof, an (OR3carboxylate or another xe2x80x94Si(OR8)3 group and combinations thereof. Examples of tetrafunctional silanes represented by the formula Si(OR7)4 are tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, tetraisopropyl orthosilicate, tetrabutyl orthosilicate, tetraisobutyl orthosilicate, tetrakis(methoxyethoxy)silane, tetrakis(methoxypropoxy)silane, tetrakis(ethoxyethoxy)silane, tetrakis(methoxyethoxyethoxy)silane, trimethoxyethoxysilane, dimethoxydiethoxysilane, triethoxymethoxysilane, poly(dimethoxysiloxane), poly(diethoxysiloxane), poly(dimethoxydiethoxysiloxane), tetrakis(trimethoxysiloxy)silane, tetrakis(triethoxysiloxy)silane, and the like. In addition to the R7 and R8 substituants described above for the tetrafunctional silane, R7 and R8 taken with oxygen (OR7) and (OR8) can be carboxylate groups. Examples of tetrafunctional silanes with carboxylate functionalities are silicon tetracetate, silicon tetrapropionate and silicon tetrabutyrate.
The multifunctional compounds which can be employed in the formulation of the coating compositions of the present invention can be any multifunctional carboxylic acid, multifunctional anhydride and combinations thereof which is compatible with the epoxy functional silane and the tetrafunctional silans of the coating compositions and which is capable of interacting with the hydrolysis products and partial condensates of the epoxy functional silane and the tetrafunctional silane to provide a coating composition which, upon curing, produces a substantially transparent, abrasion resistant coating having a Bayer number of at least 5 when employing the test method hereinbefore described.
Examples of multifunctional carboxylic acids which can be employed as the multifunctional compound in the compositions of the present invention include malic acid, aconitic acid (cis,trans), itaconic acid, succinic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexyl succinic acid, 1,3,5 benzene tricarboxylic acid, 1,2,4,5 benzene tetracarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,1-cyclohexanediacetic acid, 1,3-cyclohexanediacetic acid, 1,3,5-cyclohexanetricarboxylic acid and unsaturated dibasic acids such as fumaric acid and maleic acid and combinations thereof.
Examples of multifunctional anhydrides which can be employed as the multifunctional compound in the coating compositions of the present invention include the cyclic anhydrides of the above mentioned dibasic acids such as succinic anhydride, itaconic anhydride, glutaric anhydride, trimellitic anhydride, pyromellitic anhydride, phthalic anhydride and maleic anhydride and combinations thereof.
The nature of the interaction between the epoxy functional silane, the tetrafunctional silane and the multifunctional compound, and the effect that such interaction has on the abrasion resistance of the cured coating is not fully understood. It is believed, however, that the multifunctional compound acts as more than just a hydrolysis catalyst for the silanes. In this regard, it can be proposed that the multifunctional compound has specific activity towards the epoxy functionality on the silane. The reaction of the epoxy groups with carboxylic acids is well known and can occur under either acidic or basic conditions. The carboxylate groups on the multifunctional compound will also most likely have some activity towards the silicon atoms in the matrix; and such interaction may be through normal exchange reactions with residual alkoxide and hydroxide groups or, alternatively, through some hypervalent state on the silicon atoms. The actual interaction involving the multifunctional compound may, in fact, be a combination of all of the above possibilities, the result of which would be a highly crosslinked matrix. Thus, the matrix is enhanced through extended linkages involving the multifunctional compound.
As examples of the significance of these possible interactions, coatings prepared with non-multifunctional compounds, for example acetic acid, fail to show the same high degree of stability and abrasion resistance as obtained through the use of the multifunctional compounds. In this case, a non-multifunctional acid would have the same utility in the coating composition as a hydrolysis catalyst for the silanes, but could not provide the extended linkages presumed to be possible with the multifunctional compounds.
The coating compositions of the present invention are also very stable with respect to aging, both in terms of performance and solution stability. The aging of the coating compositions is characterized by a gradual increase in viscosity which eventually renders the coating compositions unusable due to processing constraints. Aging studies have shown that the coating compositions of the present invention, when stored at temperatures of 5xc2x0 C. or lower, have usable shelf lives of 3-4 months. During this period, the abrasion resistance of the cured coatings does not significantly decrease with time. Further, such studies have shown that stability of the coating compositions is dependent on the relative concentrations of the epoxy functional silane, the tetrafunctional silane and the multifunctional compound. In general, higher concentrations of the epoxy functional silane and the multifunctional compound contribute to increased stability of the coating mixture. Thus, in addition to providing enhanced abrasion resistance to the cured coatings, the multifunctional compound contributes to the overall stability of the coating compositions.
While the coating compositions produced by the unique combination of an epoxy functional silane, a tetrafunctional silane and a multifunctional compound provide the primary basis for the high abrasion resistance of coatings prepared by curing such coating compositions, the coating compositions may additionally include other materials to: (a) enhance the stability of the coating compositions; (b) increase the abrasion resistance of cured coatings produced by the coating compositions; (c) enhance processing of the coating compositions; and (d) provide other desirable properties of the cured coating produced from the coating compositions.
The coating compositions of the present invention may further include from about 0.1 to about 50 weight percent, based on the weight of total solids of the coating compositions, of a mixture of hydrolysis products and partial condensates of one or more silane additives (i.e, trifunctional silanes, difunctional silanes, monofunctional silanes, and mixtures thereof. The silane additives which can be incorporated into the coating compositions of the present invention have the formula R9xSi(OR10)4xe2x88x92x where x is a number of 1, 2 or 3; R9 is H, or an alkyl group containing from 1 to about 10 carbon atoms, a functionalized alkyl group, an alkylene group, an aryl group an alky ether group and combinations thereof; R10 is H, an alkyl group containing from 1 to about 10 carbon atoms, an acetyl group; and combinations thereof. Examples of silane additives represented by the above-defined formula are methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane, isobutyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane, decyltrimethoxysilane, cyclohexyltrimethoxysilane, cyclohexylmethyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, vinyltrimethoxysilane, allyltrimethoxysilane, dimethyldimethoxysilane, 2-(3-cyclohexenyl)ethyltrimethoxysilane, 3-cyanopropyltrimethoxysilane, 3-chloropropyltrimethoxysilane, 2-chloroethyltrimethoxysilane, phenethyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, phenyltrimethoxysilane, 3-isocyanopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 4-(2-aminoethylaminomethyl)phenethyltrimethoxysilane, chloromethyltriethoxysilane, 2-chloroethyltriethoxysilane, 3-chloropropyltriethoxysilane, phenyltriethoxysilane, ethyltriethoxysilane, propyltriethoxysilane, butyltriethoxysilane, isobutyltriethoxysilane, hexyltriethoxysilane, octyltriethoxysilane, decyltriethoxysilane, cyclohexyltriethoxysilane, cyclohexylmethyltriethoxysilane, 3-methacryloxypropyltriethoxysilane, vinyltriethoxysilane, allyltriethoxysilane, [2-(3-cyclohexenyl)ethyltriethoxysilane, 3-cyanopropyltriethoxysilane, 3-methacrylamidopropyltriethoxysilane, 3-methoxypropyltrimethoxysilane, 3-ethoxypropyltrimethoxysilane, 3-propoxypropyltrimethoxysilane, 3-methoxyethyltrimethoxysilane, 3-ethoxyethyltrimethoxysilane, 3-propoxyethyltrimethoxysilane, 2-[methoxy(polyethyleneoxy)propyl]heptamethyltrisiloxane, [methoxy(polyethyleneoxy)propyl]trimethoxysilane, [methoxy(polyethyleneoxy)ethyl]trimethoxysilane, [methoxy(polyethyleneoxy)propyl]-triethoxysilane, [methoxy(polyethyleneoxy)ethyl]triethoxysilane.
The selection of the silane additive, as well as the amount of such silane additive incorporated into the coating compositions will depend upon the particular properties to be enhanced or imparted to either the coating composition or the cured coating composition. For example, when the difunctional silane dimethyldimethoxysilane is utilized as the silane additive and incorporated into the coating composition in an amount of about 10% or less, based on the total solids of the composition, the viscosity increase is greatly reduced during aging of the coating composition, without greatly affecting the resultant abrasion resistance of the cured coating.
In certain applications, it is useful to add colloidal silica to the coating composition. Colloidal silica is commercially available under a number of different tradename designations, including Nalcoag(copyright) (Nalco Chemical Co., Naperville, Ill.); Nyacol(copyright) (Nyacol Products, Inc., Ashland, Md.); Snowtex(copyright) (Nissan Chemical Industries, LTD., Tokyo, Japan); Ludox(copyright) (DuPont Company, Wilmington, Del.); and Highlink OG(copyright) (Hoechst Celanese, Charlotte, N.C.). The colloidal silica is an aqueous or organic solvent dispersion of particulate silica and the various products differ principally by particle size, silica concentration, pH, presence of stabilizing ions, solvent makeup, and the like. It is understood by those skilled in the art that substantially different product properties can be obtained through the selection of different colloidal silicas.
Colloidal silica, when added to a coating composition, is considered a reactive material. The surface of the silica is covered with silicon bound hydroxyls, some of which are deprotonated, which can interact with materials in the coating composition. The extent of these interactions is dictated by a variety of factors, including solvent system, pH, concentration, and ionic strength. The manufacturing process further affects these interactions. It is thus recognized by those skilled in the art, that colloidal silica can be added into a coating formulation in different ways with different results.
In the coating compositions of the present invention, colloidal silica can be added into the coating compositions in a variety of different ways. In some instances, it is desirable to add the colloidal silica in the last step of the reaction sequence. In other instances, colloidal silica is added in the first step of the reaction sequence. In yet other instances, colloidal silica can be added in an intermediate step in the sequence.
It has been observed that the addition of colloidal silica to the coating compositions of the present invention can further enhance the abrasion resistance of the cured coating compositions and can further contribute to the overall stability of the coating compositions. The most significant results have been achieved with the use of aqueous basic colloidal silica, that is, aqueous mixtures of colloidal silica having a pH greater than 7. In such cases, the high pH is accompanied by a higher concentration of a stabilizing counterion, such as the sodium cation. Cured coatings formulated from the coating compositions of the present invention which contain basic colloidal silicas have shown abrasion resistance comparable to those of a catalyzed coating composition of the present invention (that is, a composition of hydrolysis products and partial condensates of an epoxy functional silane, a tetrafunctional silane, a multi-functional compound and a catalyst such as sodium hydroxide), but the coating compositions containing colloidal silica have enhanced stability with respect to the catalyzed compositions which do not contain colloidal silica.
In the same manner, it is also possible to add other metal oxides into the coating compositions of the present invention. Such additions may be made instead of, or in addition to, any colloidal silica additions. Metal oxides may be added to the inventive coatings to provide or enhance specific properties of the cured coating, such as abrasion resistance, refractive index, anti-static, anti-reflectance, weatherability, etc. It will be recognized by those skilled in the art that similar types of considerations that apply to the colloidal silica additions will also apply more generally to the metal oxide additions.
Examples of metal oxides which may be used in the coating compositions of the present invention include silica, zirconia, titania, ceria, tin oxide and mixtures thereof.
The amount of colloidal silica incorporated into the coating compositions of the present invention can vary widely and will generally depend on the desired properties of the cured coating produced from the coating compositions, as well as the desired stability of the coating compositions. Similarly, the amount of metal oxides incorporated into the coating compositions of the present invention can vary widely and will generally depend on the desired properties of the cured coating produced from the coating compositions, as well as the desired stability of the coating compositions.
When colloidal silica and/or metal oxides are added, it is desirable to add from about 0.1 to about 50 weight percent of solids of the colloidal silica and/or metal oxides, based on the total solids of the composition, to the coating compositions of the present invention. The colloidal silica and/or metal oxides will generally have a particle size in the range of 2 to 150 millimicrons in diameter, and more desirably, a particle size in the range of from about 2 to 50 millimicrons.
Although a catalyst is not an essential ingredient of the present invention, the addition of a catalyst can affect abrasion resistance and other properties of the coating including stability, tinting capacity, porosity, cosmetics, caustic resistance, water resistance and the like. The amount of catalyst used can vary widely, but when present will generally be in an amount sufficient to provide from about 0.1 to about 10 weight percent, based on the total solids of the composition.
Examples of catalysts which can be incorporated into the coating compositions of the present invention are (i) metal acetylacetonates, (ii) diamides, (iii) imidazoles, (iv) amines and ammonium salts, (v) organic sulfonic acids and their amine salts, (vi) alkali metal salts of carboxylic acids, (vii) alkali metal hydroxides and (viii) fluoride salts. Thus, examples of such catalysts include for group (i) such compounds as aluminum, zinc, iron and cobalt acetylacetonates; group (ii) dicyandiamide; for group (iii) such compounds as 2-methylimidazole, 2-ethyl-4-methylimidazole and 1-cyanoethyl-2-propylimidazole; for group (iv), such compounds as benzyldimethylamine, and 1,2-diaminocyclohexane; for group (v), such compounds as trifluoromethanesulfonic acid; for group (vi), such compounds as sodium acetate, for group (vii), such compounds as sodium hydroxide, and potassium hydroxide; and for group (viii), tetra n-butyl ammonium fluoride, and the like.
An effective amount of a leveling or flow control agent can be incorporated into the composition to more evenly spread or level the composition on the surface of the substrate and to provide substantially uniform contact with the substrate. The amount of the leveling or flow control agent can vary widely, but generally is an amount sufficient to provide the coating composition with from about 10 to about 5,000 ppm of the leveling or flow control agent. Any conventional, commercially available leveling or flow control agent which is compatible with the coating composition and the substrate and which is capable of leveling the coating composition on a substrate and which enhances wetting between the coating composition and the substrate can be employed. The use of leveling and flow control agents is well known in the art and has been described in the xe2x80x9cHandbook of Coating Additivesxe2x80x9d (ed. Leonard J. Calbo, pub. Marcel Dekker), pg 119-145.
Examples of such leveling or flow control agents which can be incorporated into the coating compositions of the present invention include organic polyethers such as TRITON X-100, X-405, N-57 from Rohm and Haas, silicones such as Paint Additive 3, Paint Additive 29, Paint Additive 57 from Dow Corning, SILWET L-77, and SILWET L-7600 from OSi Specialties, and fluorosurfactants such as FLUORAD FC-171, FLUORAD FC-430 and FLUORAD FC-431 from 3M Corporation.
In addition, other additives can be added to the coating compositions of the present invention in order to enhance the usefulness of the coating compositions or the coatings produced by curing the coating compositions. For example, ultraviolet absorbers, antioxidants, and the like can be incorporated into the coating compositions of the present invention, if desired.
The coating compositions of the present invention can be prepared by a variety of processes to provide stable coating compositions which, upon curing, produce substantially transparent coatings having enhanced abrasion resistance. For example, the epoxy functional silane, the tetrafunctional silane and the multifunctional compound can be added to the aqueous-organic solvent solution and stirred for a period of time effective to produce a coating composition having improved stability. When cured, such coating compositions have Bayer numbers ranging from about 6 to about 8 when employing the test method hereinbefore described. However, by incorporating a catalyst into the aqueous-organic solvent mixtures containing the epoxy functional silane, the tetrafunctional silane and the multifunctional compound, the Bayer numbers of the cured coatings produced from such coating compositions are increased so as to range from about 8 to about 15 when employing the test method hereinbefore described.
When an aqueous hydrolyzate of the epoxy functional silane is mixed with a solution of the multifunctional compound and combined with the tetrafunctional silane a coating composition is formed which when cured has a Bayer value of about 7 when employing the test method herein before described.
When a tetrafunctional silane hydrolyzate is formed in the presence of the multifunctional compound or other acid and the aqueous-organic mixture, and the epoxy functional component is added to this mixture, a coating composition is obtained which when cured provides a Bayer value of about 7 when employing the test method herein before described.
When a mixture of the tetrafunctional silane and the multifunctional compound is hydrolyzed and treated with an effective amount of sodium hydroxide and then admixed with an aqueous hydrolyzate of the epoxy functional silane, the resulting cured coating composition has a Bayer value of about 14 when employing the test method herein before described.
From the above, it becomes clear to those skilled in the art that various methods can be employed for producing the coating compositions of the present invention, and that such compositions, when cured, provide coatings having improved abrasion resistance. Further, the coating compositions have a desired stability which enhances their usefulness. However, by altering the method of preparing such compositions, product properties, such as stability and abrasion resistance, i.e., Bayer number, can be affected.
The compositions of the invention can be applied to solid substrates by conventional methods, such as flow coating, spray coating, curtain coating, dip coating, spin coating, roll coating, etc. to form a continuous surface film. Any substrate compatible with the compositions can be coated with the compositions, such as plastic materials, wood, paper, metal, printed surfaces, leather, glass, ceramics, glass ceramics, mineral based materials and textiles. The compositions are especially useful as coatings for synthetic organic polymeric substrates in sheet or film form, such as acrylic polymers, poly(ethyleneterephthalate), polycarbonates, polyamides, polyimides, copolymers of acrylonitrile-styrene, styrene-acrylonitrile-butadiene copolymers, polyvinyl chloride, butyrates, polyethylene and the like. Transparent polymeric materials coated with these compositions are useful as flat or curved enclosures, such as windows, skylights and windshields, especially for transportation equipment. Plastic lenses, such as acrylic or polycarbonate ophthalmic lenses, can also be coated with the compositions of the invention.
By choice of proper formulation, application conditions and pretreatment (including the use of primers) of the substrate, the coating compositions of the present invention can be adhered to substantially all solid surfaces. Abrasion resistant coatings having Bayer numbers of at least 5 employing the test method hereinbefore described can be obtained by heat curing at temperatures in the range of 50xc2x0 C. to 200xc2x0 C. for a period of from about 5 minutes to 18 hours. The coating thickness can be varied by means of the particular application technique, but coatings having a thickness of from about 0.5 to 20 microns, and more desirably from about 1-10 microns, are generally utilized.